1. Field of the Invention
The present invention relates generally to a low voltage, non-linear power amplifier, and specifically to an efficient, non-linear power amplifier which amplifies a constant envelope phase modulation component of a modulation signal and combines an amplitude envelope of the modulation signal with the amplified constant envelope phase modulation signal at the output of the power amplifier.
2. Description of Related Art
Digital wireless phones have gained widespread use throughout the world. These phones typically utilize cellular network systems, such as PHS, having base stations serving a predetermined area or cell, where each base station has a certain number of channels for communicating with handsets within that cell. The transmitted signals undergo some type of modulation to transmit a signal on an available channel within the predetermined cell, where the modulated signals are amplified to a desired power level for transmission. The power levels of the signals transmitted by the handsets are precisely controlled by power amplifiers in order to prevent signal interference from occurring at the base station with signals received from other handsets, while also ensuring a quality signal is transmitted. Digital modulators, such as a .pi./4 differential quadrature phase shift-keying (DQPSK) modulator, are typically used for PHS handsets and other digital wireless phones. A .pi./4 DQPSK modulated signal contains both phase and amplitude information, which requires the transmit chain of the handset to maintain a certain level of linearity in order to meet system requirements of adjacent channel power and transmit accuracy. The linearity requirements on the power amplifier require it to be operated in a linear power mode, such as a Class AB mode. Linear power modes are less efficient than other power modes and require a large amount of current to operate. Thus, the large amount of current consumed by a telephone handset using a linear power amplifier reduces the talk time of the handset or requires the use of a large battery.
Typically, data to be transmitted by the telephone handset is fed through a differential encoder, where an in-phase component (I) and a quadrature component (Q) of the modulation signal to be transmitted are created. The I and Q components are then passed through digital filters which give the modulation a particular shape. The resultant I and Q filtered signals are then modulated at a radio frequency for transmission and combined as a phase modulated signal. The phase modulated signal is then amplified to bring the signal to a desired power level for transmission.
This type of conventional power amplification requires a linear power amplifier to be used in order to maintain the requisite level of linearity, which undesirably operates the handset in a non-efficient manner. The talktime of the handset could be increased if a more efficient, non-linear power amplifier were utilized in the transmit architecture. However, the in-phase and quadrature components of the modulation signal and, thus, the phase modulated signal will have both phase and amplitude modulation (AM) components. Thus, both the phase and amplitude modulation components of the signals are fed through the power amplifier. When an AM envelope signal is amplified with a non-linear power amplifier, the AM envelope on the output of the amplifier is distorted due to the spectral spreading of the AM envelope signals which will occur in the power amplifiers. Therefore, non-linear power amplifiers can not be used with current digital telephones without causing distortion in the modulation signal due to the presence of the AM envelope.
There is a need for a non-linear power amplifier which amplifies a phase modulated signal to a desired power level without amplifying the amplitude envelope of the phase modulated signal. Moreover, there is a need for a non-linear power amplifier which reintroduces the amplitude envelope into the phase modulated signal at the output of the power amplifier in an efficient manner to maintain a low current consumption by the non-linear power amplifier.